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Class 11 Chemistry s-Block Elements NCERT Theory: Group 1 and Group 2 Properties, Reactions, Trends, Compounds, and Biological Importance


The s-Block Element – Class 11 NCERT Theory


Introduction:



The s-block elements include Group 1 (alkali metals) and Group 2 (alkaline earth metals).


These elements have their valence electron(s) in the s-orbital.


General electronic configuration:


Group 1: ns¹


Group 2: ns²


Group 1 Elements – Alkali Metals


Elements: Li, Na, K, Rb, Cs, Fr


Soft, silvery-white metals


Low melting and boiling points


Low density (Li, Na float on water)


Highly reactive, especially with water and air


Stored under kerosene



Physical Properties:


Large atomic and ionic size


Low ionization enthalpy


Good conductors of heat and electricity


Flame test: Li – crimson red, Na – golden yellow, K – violet



Chemical Properties:


Reactivity with air: Forms oxides, peroxides, or superoxides


Reactivity with water: Produces strong base (MOH) and H₂ gas



\ce{2Na + 2H2O -> 2NaOH + H2}


Important Compounds of Sodium:


NaCl – Common salt


NaOH – Strong base, caustic soda


Na2CO3.10H2O – Washing soda


NaHCO3 – Baking soda


Na2SO4.10H2O – Glauber’s salt




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Group 2 Elements – Alkaline Earth Metals


Elements: Be, Mg, Ca, Sr, Ba, Ra


Harder than alkali metals


Higher melting points and densities


Less reactive than Group 1 but reactivity increases down the group


Flame test: Ca – brick red, Sr – crimson, Ba – apple green



Physical Properties:


Smaller atomic and ionic sizes than Group 1


Higher ionization enthalpies


Form +2 oxidation state


Less electropositive than alkali metals



Chemical Properties:


Reaction with oxygen: Forms oxides (MO), some form peroxides (BaO₂)


Reaction with water:


Be does not react


Mg reacts slowly


Ca, Sr, Ba react readily forming hydroxides




\ce{Ca + 2H2O -> Ca(OH)2 + H2}


Reaction with acids: Liberates H₂ gas



Important Compounds of Calcium:


CaO (Quick lime) – Basic oxide, used in cement


Ca(OH)₂ (Slaked lime) – Used in whitewashing


CaCO₃ (Limestone) – Used in glass, cement, etc.


CaSO₄·½H₂O (Plaster of Paris) – Used in casts and building


CaCl₂ – Moisture absorbent




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Anomalous Behavior of Lithium and Beryllium:


Due to small size, high ionization energy, and high polarizing power


Lithium:


Forms covalent compounds


Li₂CO₃ decomposes easily


LiOH is a weak base


Shows diagonal relationship with Mg



Beryllium:


Amphoteric oxide and hydroxide


Forms covalent halides


Does not react with water





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Diagonal Relationship:


Li ↔ Mg and Be ↔ Al


Due to similar charge/radius ratio and electronegativity




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Trends in s-Block Elements:


Property Trend (Down the Group)


Atomic size Increases

Ionization enthalpy Decreases

Electropositivity Increases

Melting/boiling point Decreases (Group 1)

Reactivity Increases

Thermal stability Increases (carbonates)




---


Biological Importance:


Na⁺ and K⁺: Maintain osmotic balance, nerve transmission


Mg²⁺: Present in chlorophyll, enzyme activator


Ca²⁺: Bones, teeth, blood clotting, muscle function




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